This invention relates to an improvement in an ultrafiltration process for purifying solutions of polymeric anthraquinone colorants.
U.S. Pat. No. 3,920,855 of Dawson et al., issued Nov. 18, 1975, and copending patent application Ser. No. 520,530, filed Nov. 4, 1974, by Gless et al. now U.S. Pat. No. 4,018,826, issued on Apr. 19, 1977 and Ser. No. 638,730, filed Dec. 8, 1975, by Wingard et al. and Japanese patent applications of Tanabe Segaku, such as Nos. 41-14433, 41-14434 and 44-13382, disclose that colors, including anthraquinone colors, can offer advantages in polymeric form. U.S. patent application Ser. No. 727,097 filed on Sept. 27, 1976 by Ned Weinshenker further discloses that ultrafiltration is useful to purify polymeric colorants and free these colorants from undesired monomeric and low molecular weight polymeric impurities. Ultrafiltration is disclosed by that application to be applicable to polymeric colorants, generally. The present invention concerns an improvement which increases the efficiency of ultrafiltration when it is applied to purify polymeric anthraquinone colorants.